Capacitive-type sensors for measuring the level of fluids have been previously used in the art. In U.S. Pat. No. 4,176,553 to Wood, for example, there is described a capacitive sensor for sensing the level of fuel in an automotive fuel tank. The sensor includes a housing with a plurality of capacitive plates extending in corresponding arms of the housing so as to monitor liquid levels at separate locations. Each plate is coated with an insulating film to prevent shorting of the plates when fuel is present. Each of the capacitors are interrogated to derive separate signals from which a difference signal is determined. The difference signal is then compared with a predetermined maximum difference signal value to determine if the sensed levels correspond to each other within a predetermined allowable difference. If the measured discrepancy is within the maximum allowable amount, an enabling signal is generated which allows one of the capacitors to be read and used to indicate the quantity of fuel remaining in the tank.
As another example, U.S. Pat. No. 5,103,368 to Hart describes a capacitive fluid level sensor which senses level by charging a plurality of capacitors in sequence. The capacitors are formed by two concentric tubes which are vertically oriented in a tank. The vertical orientation allows the fluid to be the dielectric between the plates. As the fluid level falls, a greater area of the plates is exposed to air as the dielectric, which changes the capacitance of the capacitor. For irregular shaped tanks, the capacitors are arranged in a non-linear manner. After each capacitor is charged for a fixed time interval, the resultant voltage is compared with a known voltage to obtain an output signal representing liquid depth.
The readings obtained from prior art capacitive-type sensors are subject to variation and inaccuracies based on a number of variables. For example, a change in the dielectric constant of either the liquid being measured or the gas above the liquid can significantly effect sensor readings. Pressure and temperature changes of the liquid or gas can result in significant shifts to the dielectric constants of the liquid and gas, which introduces inaccuracies to the sensor readings. Because prior art sensors fail to compensate for such variances and inaccuracies, their usefulness is limited to applications where pressure and temperature (and therefore, dielectric constant) is substantially constant, or where precision is not a requirement. Additionally, many prior art level sensors must be calibrated at the time the sensor is installed for service, thus complicating the sensor's installation and setup.
What is needed, therefore, is a sensor which can consistently measure liquid level with a high degree of accuracy regardless of dielectric changes which may occur in the liquid or gas due to temperature changes, pressure changes, and other changes affecting the dielectric constants. The sensor should be capable of being calibrated at the factory so that calibration need not be performed when the sensor enters service.